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my stupidity and non-tail calls
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Date: 1999-12-15 (09:49)
From: Norman Ramsey <nr@e...>
Subject: my stupidity and non-tail calls
OK, I apologize for bothering everyone.  My `tail calls' are actually
sitting inside `try ... with' blocks, so I recognize that these cannot be
optimized since they are in the scope of different handlers.  I will
have to find a way to restructure my code.  A great pity, since
I like it the way it is... suggestions are welcome.

For those who wished to see the offending code, I append it, or you
can visit http://www.eecs.harvard.edu/~nr/rsync.html.


  let compress {size=size; blocks=blocks} infile =
    let blocktab = mkBlockTable blocks in
    let roll = Checksum.roll size in
    let rec compressLoop instr' b q csum infile =
      try (* first case: hit in the block table *)
        match BTab.find_all blocktab csum
        with [] -> raise Not_found
        | candidates ->
            let contents = Buffer.create size in
            let () = Queue.iter (Buffer.add_char contents) q in
            let fp = Fingerprint.string (Buffer.contents contents) in
            let (blockNum, _) = List.find (fun (_, fp') -> fp = fp') candidates in
            let instr' =
	      if Buffer.length b > 0 then STRING (Buffer.contents b) :: instr'
              else instr' in
            let instr' = BLOCK_NUMBERED blockNum :: instr' in
              ( Buffer.reset b
              ; Queue.clear q
              ; startCompressing instr' b q infile
      with Not_found ->
          let next = input_char infile  in
          let ()   = Queue.add next q  in
          let prev = Queue.take q  in
          let ()   = Buffer.add_char b prev  in
          let csum = roll csum prev next  in
            compressLoop instr' b q csum infile 
            (*********** not really a tail call ******)
        with End_of_file ->
          finishCompressing instr' b q
    and finishCompressing instr' b q = 
      let () = Queue.iter (Buffer.add_char b) q in
        List.rev (STRING (Buffer.contents b) :: instr')
    and startCompressing instr' b q infile =
      let rec fillAndSum csum k = 
            if k = 0 then csum
              let c = input_char infile in
                ( Queue.add c q
                ; fillAndSum (Checksum.append csum c) (k-1)
                ) in
          compressLoop instr' b q (fillAndSum (Checksum.string "") size) infile
        with End_of_file -> 
          finishCompressing instr' b q in